1. Field of the Invention
The present invention relates to a motorized retractor which is applied to a seat belt device for restraining, by a webbing, the body of a vehicle occupant seated in a seat of a vehicle or the like, and which can take-up the webbing such that the webbing can be pulled-out.
2. Description of the Related Art
Seat belt devices, which restrain a vehicle occupant seated in a seat by an elongated, belt-shaped webbing, are mounted in vehicles such as passenger cars and the like. Among such seat belt devices, so-called three-point-type seat belt devices are provided with a retractor (webbing retractor) which accommodates the webbing in a state in which the webbing is taken-up such that it can be pulled-out.
The retractor has a spool, at which the longitudinal direction proximal end side of the webbing is anchored and which takes-up the webbing from the proximal end side by rotating, and an urging member, such as a spiral spring or the like which urges the spool in the direction of taking-up the webbing. In a seat belt device provided with this retractor, when the webbing is applied to a vehicle occupant, the spool is urged by the urging force of the urging member in the direction of taking-up the webbing. The webbing, from which slack has been removed, thereby restrains the vehicle occupant. Further, when the vehicle occupant cancels the state in which the webbing is applied to him/her, the webbing is taken-up onto the spool by the urging force of the urging member.
Here, if the urging force of the urging member is small, the webbing is in a slack state in which it is not completely taken-up, which is a cause of deterioration in appearance at times when the webbing is not in use. On the other hand, if the urging force of the urging member is large, it is a cause of imparting a constricting sensation to the occupant to which the webbing is applied.
Therefore, a motorized retractor (motorized seat belt retractor) having a mechanism which drives the spool by the driving force of a motor (a so-called “take-up assisting mechanism”) has been conceived of in order to reduce the urging force of the urging member and mitigate (suppress) the constricting sensation imparted to the vehicle occupant, and in order to compensate for the reduction in the force taking-up the webbing onto the spool due to this reduction in urging force.
In a motorized retractor equipped with such a take-up assisting mechanism, in consideration of the comfort and the like for the vehicle occupant, it is preferable that the take-up torque of the spool by the driving force of the motor be set to be low to the extent of supplementing the urging force of the urging member. To this end, a reduction mechanism whose reduction ratio is set to be low is provided between the output shaft of the motor and the spool.
On the other hand, when the urging force of the urging member is decreased so as to mitigate the constricting sensation imparted to the vehicle occupant as described above, a slight amount of looseness known as “slack” arises at the webbing in its applied state, and is a cause of a deterioration in the restraining performance of the webbing at the time of a vehicle collision or the like.
Therefore, a motorized retractor has been conceived of which is equipped with a mechanism (a so-called “pretensioner mechanism”) which improves the restraining performance of the webbing by forcibly rotating the spool in the take-up direction by the driving force of a motor when the risk of a collision is sensed.
In a motorized retractor equipped with such a pretensioner mechanism, the webbing must be forcibly taken-up against the inertial force of the vehicle occupant who starts to move toward the front of the vehicle due to sudden deceleration of the vehicle. Therefore, the take-up torque of the spool by the driving force of the motor must be set to be high. Thus, a reduction mechanism whose reduction ratio is set to be high is provided between the output shaft of the motor and the spool.
Further, a motorized retractor is known which provides both a take-up assisting mechanism and a pretensioner mechanism as described above by a single motor (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-63522).
In the motorized retractor disclosed in JP-A No. 2001-63522, two different reduction mechanisms, which can transfer the rotational force of an output shaft of a motor to a spool, are provided between the output shaft and the spool. These reduction mechanisms are set to different reduction ratios. When one of these reduction mechanisms is switched to by a switching mechanism which includes a solenoid, the rotational force of the output shaft is transferred to the spool via the reduction mechanism which has been switched to.
Accordingly, when the reduction mechanism whose reduction ratio is set to be low is switched to, the spool is rotated at high speed and low torque. When the reduction mechanism whose reduction ratio is set to be high is switched to, the spool is rotated at low speed and high torque. In this way, the mutually contradictory performances demanded of the take-up assisting mechanism and the pretensioner mechanism can both be achieved by a single motor.
However, in the motorized retractor of the above-described structure, because the switching mechanism is a complex structure including a solenoid as described above, space for placement of the solenoid must be ensured, which is a cause of the device becoming large.